The invention relates to supercharged internal combustion engines, more particularly supercharged Diesel engines, comprising a compressor supplying fresh air in parallel to the engine and to a bypass having an auxiliary combustion chamber and a turbine which receives the engine exhaust gases and the gases from the auxiliary combustion chamber and mechanically drives the compressor, the bypass being divided into two main branches, the first of which ends in a dilution region or "secondary region" downstream of the upstream part or "primary region" of the auxiliary combustion chamber and has throttle means having a variable flow cross-section, whereas the second branch starts from a place on the first branch upstream of the first throttle means and opens into the primary region via second throttle means for throttling the flow cross-section, the second throttle means comprising coupled passages having a variable common free cross-section and formed respectively in an internal and external cylindrical means which are movable relative to one another, one of the cylindrical means at least partly bounding the primary region whereas the other at least partly bounds a cavity directly connected to the compressor outlet, at least one fuel injector opening into the primary region in the immediate neighbourhood of the aforementioned coupled passages, means being provided for correspondingly varying the flow rate of the fuel injector or injectors and also varying the flow rate of air entering the primary region via the common free section of the coupled passages, by moving the inner cylindrical means relative to the outer means.
Engines of the aforementioned kind have been described in U.S. Pat. No. 4,026,115. The coupled passages or orifices acting as the second throttle means by aerodynamic valve regulation (or blockage of air streams by partial alignment of pairs of coupled passages or orifices) have a position relative to the injector or injectors introducing fuel into the primary region of the auxiliary combustion chamber and relative to the first throttle means such that they produce a pressure drop which directly acts on the coupled passages or orifices. The result, in the primary region, is strong turbulence which provides optimum conditions for combustion irrespective of the extent to which the coupled passages or orifices are opened, i.e. under all operating conditions of the auxiliary combustion chamber. This saves fuel and prevents problems being caused by deposits of soot or coke on the walls of the auxiliary combustion chamber. The pressure drop can also be used for permanently and efficiently producing adequate air streams ("film cooling") via at least some of the constant-section air passages forming part of the second throttle means, so as inter alia to cool the walls of the primary region of the auxiliary combustion chamber under all conditions.
Note that the inner and outer cylindrical means, which are generally sleeves, can move relative to one another in translation or rotation or both, at least one of the inner and outer cylindrical means being movable. Usually the outer cylindrical means is stationary whereas the inner cylindrical means is movable, preferably in translation.
Since the outer cylindrical means is scavenged by the air supplied by the compressor, the air tends to maintain the outer cylindrical means at a relatively low, constant temperature. On the other hand the inner cylindrical means, which at least partly bounds the primary region of the auxiliary combustion chamber, is brought to a relatively high temperature which varies with the flow rate of fuel introduced through the injector or injectors and burnt in the primary region. The variations in temperature can reach e.g. 600.degree. C. during operation. The inner and outer cylindrical means or sleeves are made of a material (refractory stainless steel) which expands considerably with temperature (of the order of 2 mm per meter and per 100.degree. C. temperature rise).
The radial clearance between the two cylindrical means when cold has to be greatly increased to prevent them jamming together with hot (i.e. when the inner cylindrical means is hotter, corresponding to the maximum flow rate of air and fuel in the primary region). However, during pilot operation of the auxiliary combustion chamber (minimum flow rate of fuel and air in the primary region and combustion restricted to the bottom of the chamber), the inner cylindrical means radially contracts during cooling and leaves an annular space between its outer wall and the inner wall of the outer cylindrical means. The cross-section of the space is very large (of the order of 700 mm.sup.2 when the diameter of the cylindrical means or sleeves is about 200 mm, under the aforementioned conditions with regard to temperature variations and the expansion coefficient). The annular space allows air to leak. The leakage is in proportion to the supercharging pressure, since the density of air is related to pressure and the pressure difference between the two sides of the leak cross-section increases with the supercharging pressure. Now it is precisely under such high-pressure conditions that the engine is most heavily loaded and must therefore be most effectively scavenged. No success has hitherto been obtained in attempts to reduce the aforementioned leak, inter alia by placing labyrinths at the periphery of the inner cylindrical member.
We are therefore faced with the following alternatives: if the radial clearance is small, the movable cylindrical means will jam against the stationary cylindrical means at the full flow rate, so that the chamber will unnecessarily be held in the full flow-rate position (resulting in overheating and fuel waste), or if the radial clearance is large, the auxiliary combustion chamber leaks during pilot operation. In that case, the first throttle means will be incapable of controlling the pressure drop between the compressor and turbine; this will interfere with proper scavenging of the engine at high power, as already explained, unless the compressor flow rate is increased, which will have the disadvantage of adversely affecting specific consumption under partial load.